Accepted Summary

1. The air speeds and sinking speeds of birds gliding at equilibrium fall in a performance area when these quantities are plotted against one another. Three curves bound the performance area: (i) a curve for minimum sinking speed at each air speed, (ii) a curve for maximum sinking speed at each air speed, and (iii) a curve dependent on the maximum lift coefficient of the wings. I have discussed curve i in a previous paper. This paper discusses the theory of curves ii and iii, which describe rapid descent in gliding birds. 2. I used an optical tracking device (an ornithodolite) to measure air speeds and sinking speeds of 16 African white-backed vultures (Gyps africanus Salvadori) descending rapidly from altitudes 200-500 m above the ground. The ornithodolite measured the polar coordinates of a bird's position in space (relative to the ground) and recorded them on magnetic tape. 3. The vultures had air speeds between 5-4 and 39-lms"1, and sinking speeds between 0-2 and 8-3ms"1. Most of the observations fell within the theoretical boundaries of the performance area. 4. These data are consistent with a maximum lift coefficient of 2-2 for the wings of white-backed vultures